The present invention relates to an improved magnetic compass for marine and/or land use which has a semispherical or spherical float body with an appropriate size magnet attached to the bottom center thereof floated stably and freely inwardly of an inner circumference of a cylindrical instrument body by a rectified air current blown upwardly through the inside of an inner cylinder of said cylindrical instrument body, whereby an azimuth pointer on the surface of said semispherical or spherical float body is freely movable to indicate the geomagnetic North direction.
Magnetic compasses can generally be divided into two categories based on structural feature: one is a dry type and the other is a liquid type. Most conventional dry type compasses are inferior in the ability to move freely to the liquid type compasses because the vibration absorbing mechanism thereof is imperfect, and as a consequence such compasses are almost completely out of use at present. For this reason, liquid type compasses have become popular. A typical conventional liquid type compass is illustrated in FIG. 5 and will be described for purposes of comparison.
In FIG. 5, reference numeral 1 designates a float provided with a magnetic pointer which is freely swingably pivoted on a cap 4 by means of a pivot 3 on the central axis of the instrument body 8. Reference numeral 2 designates a compass card with a graduated azimuth scale thereon and fixed on the float 1. Reference numeral 5 designates the lubber's line plate which indicates the bow direction of a vessel and is attached to the instrument body 8. Reference numeral 9 designates a cover for the instrument body and is made of a transparent glass. The inside of the instrument body 8 is filled with a body of liquid in order to prevent the compass card from vibrating. Numeral 6 designates a regulator to properly compensate for the expansion and contraction of the liquid caused by a change of temperature. Numeral 7 designates a lead ballast to stabilize the instrument body 8 by providing weight in its bottom portion.
Although the conventional liquid type compasses generally comprise such elements, most of them have the following drawbacks:
1) When the helm of a vessel carrying such a compass is put hard port or starboard, the liquid inside the instrument body 8 comes out a concomitant movement which causes the float 1 and the compass card 2 to turn together with the ship's turning motion. As a result, the compass cannot indicate correctly a new bow direction immediately after the turning operation.
2) There is a possibility that the compass card 2 will become discolored after long periods of use.
3) In an extremely cold region, there may be times when the liquid will freeze.
4) In the tropics, there may be cases where the glass cover 9 breaks due to the expansion of the liquid, or the liquid decomposes due to the heat.
5) The method of correcting a possible variation between the azimuth scale and the actual indication by the pointer is carried out by adjusting an iron ball on the outside. However, this adjustment requires considerable labor because the structure is large and heavy.
6) The indication mechanism has an intricate structure and is heavy, so that the gimbal for supporting the compass mechanism becomes large and massive. This results in an increase in the production cost, and there is no increase in the followability of the compass proportional to the increase in the production cost.